Selenium rectifier



1945- w. E. BLACKBURN 2390 771 SELENIUM RECTIFIERS Filed July 25, 1942 5 (5 1 1 ggggy INVENTOR NITNESSESZ ayne 51ackb urn.

ATTORN Patented Dec. 11,1945

UNITED ."STATES PATE n SELENIUM RECTIFIER Wayne'E. Blackburn, Wilkinsburg, Pa., assignorfi to Westinghouse Electric Corporation,

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I Pittsburgh, Pa.', a corporation of Pennsylvania. Application July 25, 1942, Serial No. 452,263.:

' i'ciaims. (01.175-366) My invention relates to selenium rectifiers and, in particular, to methods of insuring consistent rectifying performance throughout the area of a relatively large selenium surface.

One object of my invention is to provide a rectifier of the selenium type which shall exhibit rectifying properties of great uniformity as between different specimens manufactured.

Another object of my invention is to provide a selenium rectifier which shall have a rectifying contact layer exhibiting great uniformity in rectifying action throughout its expanse.

Still another object of my invention is to produce selenium rectifiers by a wholesale manufacturing process yielding a product of great uniformity and reliability.

A further object of my invention is to produce selenium rectifiers of highelectrical emciency.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing in which the single figure illustrates a rectifier produced in accordance with the principles of my invenion.

It is customary in manufacturing selenium rectifiers to coat a backing-plate of metal with a layer of molten selenium and to subject this layer to an annealing process at a temperature somewhat below its melting point. When this is done, it is found that a layer appears on the outer surface of the selenium which may be an oxide of selenium, but which at any rate has undesirable qualities in the rectifier. For example, such a layer is liable to be found of non-uniform character throughout the extent of the surface, and also to show considerable variations between rectifiers made at different times.

In'order to avoid the difficulties resulting from this lack of uniformity and to produce rectiflers of reliable rectifying characteristics, I have found that a superior product is produced if the selenium surface, after annealing, is submerged in boiling water for thirty seconds. Thereupon the selenium element is removed from the water and permitted to dry by evaporation of the remaining moisture due to the heat stored in the element. The element is then dipped for thirty seconds into a boiling solution of sodium hydroxide of approximately 1.3 grams per liter of solution. Thereafter it is removed and permitted to dry by its own heat. The selenium unit to which the above-mentioned treatment is applied is preferably formed by taking a piece of sheet metal of the iron group such as cold-rolled steel, indicated at l in the drawing. One surface of this sheet is roughened to a matt surface by any convenient process, such as sand blasting or acid etching. On this surface I place a coating of amorphous selenium by heating it to 'a'temperature between 220C. and 250 C., the molten selenium being applied to a thickness of several mils and thereafter the plate is allowed to cool quickly. Thereafter the coated plate is heated to approximately C. and is pressed against a smooth flat surface with sufflcient force to reduce the plastic selenium to a smooth uniform layer preferably about 2 mils thick. The smooth flat surface may conveniently comprise a plate of the phenol condensation product sold under the trade name of Micarta. The plate is then introduced into an oven at room temperature and heated gradually to approximately 180 to C. within a period of two hours. remaining at this temperature for several hours, preferably at least six. The plate is then preferably gradually cooled to room temperature before the plate is removed from the oven, the time taken for cooling being preferably of about three hours.

As an alternative to insure a smooth and uniform coating of selenium, the disk with the molten layer applied may be rotated about an axis passing through its center so that centrifugal force will tend to carry a portion of the molten selenium to the periphery, surface tension maintaining a. desirable uniform thickness in the layer remaining on the major area of the plate. The selenium layer appears at 2 in the drawing.

After the treatment with sodium hydroxide mentioned above, a contact layer 3 is formed on the selenium surface by any method well known in the art for this purpose. For example, an alloy comprising approximately 75% tin and 25% cadmium may be Schoop-sprayed in an approximately uniform layer over the selenium surface, as disclosed in E. D. Wilson Patent No. 2,193,610, for Selenium contact electrodes, assigned to the assignee of the present application and issued March 12, 1940. The remaining steps in the proc ess of manufacture described in the aforesaid Wilson patent may likewise be applied to the unit chemically treated as above described, if so desired.

However, one step in the process of manufac ture which I have found to yield exceptionally good results consists in a certain treatment of the rectifier disc after the alloy coating described in the preceding paragraph has been applied. In order to insure a high resistance of the rectifier in the nonconductive direction, I impress a voltage tending to cause current flow in the high resistance direction; that is to say, high voltage tending tocause current flow in the opposite direction from that which the rectifier will allow to freely pass when alternating voltage is impressed upon it. Before impressing this voltage, however, I immerse the rectifier disc, held between appropriate electrodes, in thin transformer oil for a period preferably of minutes duration. After such a treatment, the rectifier may be exposed to moisture without affecting the leakage current flowing in the high resistance direction and may even be stored for considerable times in a-humid atmosphere. This process likewise tends to maintain a constant and uniform temperature on the rectifier. element during the above-mentioned voltage-forming period.

While I have described the particular alkali solution employed as sodium hydroxide, it will be recognized by those skilled in the art that other chemically similar materials innormal solution, such as potassium hydroxide,v may be substituted therefor.

I claim as my invention:

1. The method of forming arectifying surface on selenium which comprises annealing the selenium at a temperature'not far below its melting point, immersing the selenium surface in boiling water and thereafter immersing it in 'a boiling solution of alkali hydroxide.

2; The method of forming a rectifying surface on selenium which comprises'annealing the selenium in air at a temperature not far below its melting point, immersing the selenium surface in boiling water and thereafter -immersing it in a boiling solution of alkali hydroxide.

3. The method of forming a rectifying surface on selenium which comprises annealing the selenium at a temperature not far below its melting point for a period of the order of hours, immersing the selenium surface in boiling water and thereafter immersing'it' in a boiling solution of alkali hydroxide.

4. The method of forming a rectifying surface on selenium which comprises annealing the selenium at a temperature not far below its melting point for a period of the order of hours,immersing the selenium surfacein boiling water "and thereafter immersing it in a boiling solution of sodium hydroxide in normal solution.

WAYNE E. BLACKBURN. 

